Abstract

Considering the function as a real function of real variable, what is its minimum value? Surprisingly, the minimum value is reached for a negative value of . Furthermore, considering the function , and , two different expressions in closed form for the inverse function can be obtained. Also, two different series expansions for the indefinite integral of and are derived. The latter does not seem to be found in the literature.

1. Introduction

Let us consider the following real function of real variable, :
and let us pose the following questions.(1)What is the minimum value of ?(2)Can its inverse function be expressed in closed form?(3)Is its indefinite integral known?

The function is termed as self-exponential function in [1, Section 26:14] and coupled exponential function in [2, Equation 01.20], using in the latter the notation . Probably, the most well-known property of is just its great growth rate. In fact, the rate of increase of as is greater than the exponential function or the factorial function [3, Chapter I. section 5]

Regarding the domain of , in [1, Section 26:14], is only defined as a real function for positive values of , and [2, p. 10] states that, for arguments less than zero, is complex except for negative integers. However, [4] says that, for , is only defined if can be written as , where and are positive integers and is odd. We will use this fact later on in order to answer the first question.

This paper is organized such that each section is devoted to each of the questions raised above.

2. Minimum Value

The usual way to answer the first question is just to solve the equation ; that is,
so

Since
then (4) is a local minimum. Moreover, since there are no more local extrema and is a smooth function, (4) is the global minimum; thus,

Nonetheless, this reasoning fails, because it does not take into account negative values of . Therefore, we need to define first the domain of for negative values of . Despite the fact that this is essentially done in [4], in order to answer the first question, we provide the following derivation.

2.1. Domain of

Let us consider first the case , where the function does not exist. However, applying L’Hôpital’s rule, the following limit is finite:

Nevertheless, the right derivative of at is infinite:

For , we may rewrite by using the signum function as

Now, for and , we have
where is the Heaviside function. Therefore, by applying Euler’s formula , we obtain

Since is a real function, the complex part of (11) has to be zero. For , is never complex, and for the complex part of is zero when

Therefore, substituting (12) into (11), the function is given by
and its domain is

Notice that, despite the fact we have considered the function as a multivalued function in (10), is a single-valued function in (13), because we are considering as a real function. Figure 1 shows the plot of . According to (13), for , the plot of lies on the following curves:
with a numerable infinite number of points. Notice that + and − signs in (15) occur for even and odd positive integers in (13), respectively.

Figure 1: Plot of as a real function of real variable.

2.2. Minimum Value of

In order to calculate the minimum value of , for , let us solve the equation ; that is,
Thus,

Notice that
So, has a maximum and has a minimum in (17), which agrees with Figure 1. However, , so we have to get the best rational approximation to in such a way that . Moreover, since the minimum lies on the curve, and must be both odd positive integers. In order to do so, let us consider the sequence
where denotes the floor function. Notice that the numerator and the denominator of (19) are both odd, so irreducible, with being odd positive integers. Therefore, is a sequence of rational numbers for which lies on the curve . Also, is a monotonic decreasing sequence that satisfies

Defining now the rational number, ,
Then, the minimum value of is
which is different from (6), as aforementioned.

3. Inverse Function

About the second question, a closed form expression for the inverse function does not seem to be found in the usual literature (see [2, Chapter 2]). However, by using the Lambert function [5], is very easily inverted. The Lambert function is defined as the inverse function of and it is implemented in MATHEMATICA by the commands ProductLog or LambertW. The Lambert function is a multivalued function that presents, for real arguments, two branches: (principal branch) and . Figure 2 shows the plot of both branches.

Figure 2: Branches of the Lambert function for real arguments.

Let us consider now on a little more general function than (1), but, for simplicity, only for positive arguments; that is,

Figure 3 shows the plot of for different values of . It is easy to prove that
which agrees with Figure 3.

Applying now the Lambert function and taking into account (26), we obtain
and thus

According to Figure 3, notice that, depending on the value of , sometimes is a double-valued function, so we have two real values of the Lambert function in (29), that is, and . In this latter case, we have used the notation . Also, from Figure 3, we can see that is a single-valued function for when and for when . Therefore, taking into account (24) and Figure 3, we can consider the following cases.(i)Consider ,
(ii)Consider ,
where we have defined

Curiously, (33) is just the closed expression given in [6] for the following power tower:
which converges if and only if

In order to see this, consider the power tower
which converges if and only if [7, 8]

Taking in (36) the logarithm of both sides and plugging back the function definition, we obtain

Performing now the change of variables , we get

Now, by using the principal branch of Lambert function, we can solve, for ,
but, from (34) and (40), we arrive at (33):
which, according to (37), converges if and only if
that is, the interval given in (35).

In [2, Equation 02.03], is termed as coupled root function. Since the latter reference is unaware of the closed expression (33), it performs numerically the following limit [2, Equation 02.07]:
in order to show that goes to infinity at a lower rate than logarithms. In fact, (43) is quite easily proved applying (33) and the property
so that

In order to compute the lower incomplete gamma function given in (55), we may use [1, Equation 45:4:1]
where is the exponential polynomial and it is given by the power-series expansion of the exponential function by truncation after the th term [1, Equation 26:12:2]:

Therefore, we finally obtain
where , and .

Notice as well that, for , according to (31), we have to calculate
but, according to (58), we have
where , and .

4.2. Indefinite Integral of

The indefinite integral
cannot be expressed in terms of a finite number of elementary functions [10]. Moreover, closed expression for (61) in the usual literature does not seem to be found. However, it can be expressed in closed form by using the upper incomplete gamma function [9, Equation ]:

Notice that if and is a positive integer, then we recover the usual gamma function:

Expanding in power-series the exponential function given in (64) and integrating term by term, we get

Performing now the change of variables , we obtain

By using now the definition of the upper incomplete gamma function (62), we arrive at

In [11, Lemma 10.6], we find a similar expression for the indefinite integral of the power tower

Taking in (67) and using (63), we recover the expression given by [12, Equation 3.342]:

Moreover, taking , in (69), we recover the expressions given for the sophomore’s dream [13, pp. 4, 44], discovered in 1697 by Johann Bernoulli, as follows:

In order to compute the upper incomplete gamma function given in (67), we may use [1, Equation 45:4:2]:
where is the exponential polynomial (57). Therefore, (67) can be rewritten as

Let us now proceed to give another expression for the indefinite integral of by using the results given in (58) and (60). First, let us consider the cases and , splitting (64) into two summands, as follows:
where the first integral in (73) has been substituted by (69) and the second integral can be computed by knowing that is an increasing function for when .

Indeed, according to Figure 4, we have
So, from (73) and (74) and taking into account (58), we get

Figure 4: Scheme for the integration of , , beyond .

Since the following series is a telescoping series:
we can simplify (75), obtaining
where , and .

Therefore, substituting (78) in (73) and taking into account (60) and (76), we arrive again at (77), but for . Moreover, the range can be extended up to the point where is a monotonic increasing or decreasing function. So, according to (24), we can say that . Then, collecting all these results, we can conclude that
where , , and .

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.